Attenuation compensation for ultrasonic thermometers

ABSTRACT

A current source and control circuit are used to supply a DC bias current to the driver coil of an ultrasonic thermometer. The magnitude of the ultrasonic echo signal is thereby increased by more than a factor of two and the upper temperature sensing limit, which is due in large part to sound attenuation in the sensor, is raised.

lb Tinned States Patent [191 [I 11 swam Shepard et a1. June 12, 1973 1ATTENUA'IION COMPENSATION FOR 3,572,097 3/1971 Kleesattel 73/67.1

ULTRASONIC THERMOMETERS FOREIGN PATENTS OR APPLICATIONS [75] Inventors:Robert L. Shepard; Albert H. 1,178,385 1/1970 Great Britain 73/339 AMalone, both of Oak Ridge, Tenn. OTHER PUBLICATIONS 1 1 Assigneer TheUnited States of lfnrwrica as NAsA Tech. Brief; Brief No. 68-10319;August 1968,

"Presented by h United States Ultrasonic Temperature Measuring Device.Atomic Energy Commission, Washmgton, Primary Examiner-Richard C.Queisser [22] Filed; Apr 2 1972 Assistant Examiner-Denis E. CorrAttorney--Ro1and A. Anderson [21] Appl. No.: 247,532

[57] ABSTRACT [52] US. Cl. 73/339 A, 181/.5 J A current source andcontrol circuit are used to supply [51] Int. Cl. G01k 11/24 a DC biascurrent to the driver coil of an ultrasonic [58] Field of Search 73/339A, 194 A, thermometer. The magnitude of the ultrasonic echo73/69:70:67.2, 67.8; 181/.5 J signal is thereby increased by more than afactor of two and the upper temperature sensing limit, which is due [56]References Cited in large part to sound attenuation in the sensor, is

UNITED STATES PATENTS raised- 3,350,942 11/1967 Peltola 73/67.8 3Claims, 2 Drawing Figures 1Q rRAusmTTzR i" RECEIVER 1 '11] (I111 i. I] 1T1 T2 Ii R1 R2 B-ECHO i f i 11 Y CONTROLLED DC CURRENT 22 SUPPLY 25\ D-1 Q B a I'VVQL'X PATENTEDJUN u 2:915

SHEHZBFZ Cu vkzmmmzo w m 200 0v om ATTENUATION COMPENSATION FORULTRASONIC THERMOMETERS BACKGROUND OF THE INVENTION This invention wasmade in the course of, or under, a contract with the United StatesAtomic Energy Commission.

The prior art ultrasonic system utilized in the present invention is thethin wire pulse-echo instrument called the Panatherm S010, manufacturedby Panametrics, Inc., and includes all of the structure illustrated inFIG. 1 of the drawings except the DC feed-back circuit consisting of theunits 21 and 22 and the connecting wires.

Prior to the present invention it was desired to apply pulse-echo,thin-wire ultrasonic thermometry to in-pile fuel temperaturemeasurements in high temperature nuclear reactors. At the temperaturesto be measured (around 2,lC and higher) improved techniques are neededsince the above prior art system could measure temperatures only up toabout 2,l00C.

The well known problem, occurring in most sensor wires, is that of soundattenuation at high temperatures. Mechanical effects such as annealing(softening), which typically begin to occur at about two-thirds of themelting point temperature, causes the reflected pulses to decrease inamplitude. When this happens,

- the amplitude drops below a minimum detection level,

making it no longer possible to time the pulse separation and hence tomeasure temperature. In some cases the receiver gain can be turned up,or a higher driving voltage used, but generally there are otherlimitations such as distortion of the pulse shape that preventincreasing the gain or driving voltage.

It is generally known that permanent magnet biasing of themagnetostrictive transducer is capable of increasing the signal-to-noiseratio at small driving voltages. This technique consists of locating asmall magnet near the magnetostrictive alloy to increase itsmagnetization. This practice is beneficial in increasing the temperaturemeasuring limit of the thermometer by increasing the echo amplitudes athigh temperatures, but it would produce signals too large at lowertemperatures which would overload the amplifier and distort the pulseshape producing a false time interval determination. In nuclear reactorsit may happen that the radiation environment or temperature in thevicinity of the transducer precludes the use of a permanent magnet orspace considerations may not allow its use.

Thus, there exists a need for another technique for increasing theuseful temperature measurement range of the above-mentioned prior artinstrument. The present invention was conceived to meet this need in amanner to be described below.

SUMMARY OF THE INVENTION It is the object of the present invention toprovide a means for increasing the useful temperature measurement rangeof a thin-wire, pulse-echo ultrasonic system.

The above object has been accomplished in the present invention by amodification of the abovementioned prior art instrument which consistsof providing a DC current supply within a feedback circuit that adds acontrolled DC magnetic bias to the transducer coil of the instrument ina manner to be de scribed hereinbelow with the result that the usefultemperature measurement range of the instrument is increased.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration of thepresent invention, and FIG. 2 is a graph showing the change in echoamplitude for various values of coil bias current.

DESCRIPTION OF THE PREFERRED EMBODIMENT The system of FIG. 1 illustratesthe basic parts of a pulse echo type, thin-wire ultrasonic thermometer.Reference numeral 15 indicates a combined transmitter/receiver having adigital display. The transmitter terminals T1 and T2 are shown connectedfor energizing the transducer coil 17. The coil 17 also functions as thereceiving coil by means of the connections to the receiver terminals R1and R2, and by means of the blocking diode 16. The coil 17 is woundaround a magnetostrictive transducer 18 that is in turn joined to thesensor wire 20 by means of the lead-in wire 19. The transducer 18 ismounted on a damping pad 24.

The system described thus far is a prior art instrument called thePanatherm 5010, manufactured by Panametrics, Inc., and operates in thefollowing manner. An electrical driving pulse (main bang pulse) from thetransmitter is applied to coil 17 and thereby energizes the transducermaterial 18 (usually a magnetostrictive alloy such as Remendur),launching ultrasonic stress pulses left and right. It should be notedthat Remendur is a metal alloy consisting of 49 percent iron, 49 percentcobalt, and 2 percent vanadium. It is manufactured by Wilbur B. DriverCompany, Newark, NJ. The ultrasonic pulse propagating along the lead-inwire 19 is partly reflected from the beginning (juncture) of the sensor20 and partly from the end (tip) of the sensor. These two sensor echoesare termed the A- echo and B-echo pulses, respectively. They return viathe wire 19 with a time interval that is a function of the sensortemperature. These reflected pulses each create a current pulse in thecoil 17 and the receiver measures the time interval between the twopulses and produces the digital display.

As an added convenience for sound attenuation studies, the peakamplitudes of the pulse pair A and B are sampled and held in separatechannels, and made available as DC voltage levels at panel terminals.Only the terminal for the B-echo value is shown in FIG. 1, and it isused in the present invention in a manner to be described hereinbelow.As mentioned above, this prior art instrument when used for measuringin-pile fuel temperatures in high temperature nuclear reactors waslimited to the measurement of temperatures up to about 2,l00C due to thesound attenutation effect of the sensor material at high temperatures.

In order to overcome the above problems and permit the measurement oftemperatures higher than 2,l00C, the above prior art instrument has beenmodified to accomplish this goal in the following manner.

An attenuation compensation circuit has been added to the aboveescribedprior art system. This compensation circuit consists of a DC currentsupply within a feedback circuit that adds a controlled DC bias currentto the transducer coil 17. As shown in FIG. 1, this compensation circuitincludes a controlled DC current supply 22 which is connected at itsoperational amplifier input to receive the B-echo DC voltage level fromthe ultrasonic receiver 15. The output of the current supply 22 isconnected in parallel with the transmitter to the driver coil 17. Achoke 21 is used in one of the output leads to isolate the supply 22from the driving pulses from the transmitter 15.

The operation of this feedback circuit is automatic. As a decrease inthe B-echo amplitude is sensed, the voltage input to the supply 22decreases. The supply 22 responds by increasing the DC current bias tothe coil 17, thus increasing the magnetization of the transducer 18, andthereby tending to prevent diminishment of the echo amplitudes. Inaddition, a good signal-to-noise ratio is retained, thus extending theusable temperature measurement range of the instrument. In practice, theultrasonic thermometer using DC current biasing, as shown in FIG. 1, maybe operated using optimum receiver gain without pulse shape distortionat low sensor temperatures, and at higher sensor temperatures,increasing the DC coil current bias to boost the received signal withoutrequiring an increased receiver gain.

A graph showing the effect of changes in the coil bias current on theB-echo amplitude is shown in FIG. 2. The main bang pulse amplitude fromthe transmitter is equal to 30 volts peak, the pulse width is equal to 3[.LSCC., the pulse repetition rate is 120 pps, and the shunt capacitance25 was not utilized in the operation of the system of FIG. 1 forobtaining the data for plotting the curve of FIG. 2. The magnitude ofthe ultrasonic echo signal is seen to be increasable by more than afactor of two.

It has been determined that the use of the DC bias current feedbackcircuit in FIG. 1 when used in conjunction with the often used impedancematching shunting capacitor 25 will result in an increase in the usefultemperature measurement range of the system to several hundred degreesabove the normal ultrasonic attenuation threshold at about 2,l00C.

This invention has been described by way of illustration rather thanlimitation and it should be apparent that it is equally applicable infields other than those described. For example, it ma/be used inultrasonic milling and welding applications.

What is claimed is:

1. In an ultrasonic thermometer system including a transducer coilencompassing a magnetostrictive transducer, a sensor wire coupled sosaid transducer, and a transmitter/receiver unit coupled to saidtransducer coil for supplying driving pulses to said coil and receivingtime-separated echo signals from said sensor wire as a function of thetemperature in the vicinity of said wire, the improvement comprising aDC current supply connected as a feedback circuit across said coil foradding a controlled DC bias thereto, said feedback circuit including achoke therein, said DC supply being connected to and controlled by oneof said echo signals received by said transmitter/receiver toautomatically adjust the DC current bias output of said supply to saidcoil, thereby substantially increasing the upper temperature sensinglimit of said system.

2. The system set forth in claim 1, wherein the transmitter portion ofsaid transmitter/receiver unit supplies to said coil a main bang pulseamplitude of 30 volts peak with a pulse width of 3 usec. and at a pulserate of I20 pps.

3. The system set forth in claim 1, wherein an impedance matchingshunting capacitor is connected across the coupling between saidtransmitter/receiver and transducer coil.

1. In an ultrasonic thermometer system including a transducer coilencompassing a magnetostrictive transducer, a sensor wire coupled sosaid transducer, and a transmitter/receiver unit coupled to saidtransducer coil for supplying driving pulses to said coil and receivingtime-separated echo signals from said sensor wire as a function of thetemperature in the vicinity of said wire, the improvement comprising aDC current supply connected as a feedback circuit across said coil foradding a controlled DC bias thereto, said feedback circuit including achoke therein, said DC supply being connected to and controlled by oneof said echo signals received by said transmitter/receiver toautomatically adjust the DC current bias output of said supply to saidcoil, thereby substantially increasing the upper temperature sensinglimit of said system.
 2. The system set forth in claim 1, wherein thetransmitter portion of said transmitter/receiver unit supplies to saidcoil a main bang pulse amplitude of 30 volts peak with a pulse width of3 Mu sec. and at a pulse rate of 120 pps.
 3. The system set forth inclaim 1, wherein an impedance matching shunting capacitor is connectedacross the coupling between said transmitter/receiver and transducercoil.